Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

If there is no such thing as velocity except
with respect to a
reference object, how can there be speed limits in deep
space?

Because deep space is not empty. There are always distant
galaxies
and stars you can use to set up a frame of reference. Even locally,
you can
measure how far away you are from your laboratory, planet or asteroid.
Special
relativity only says that ABSOLUTE velocities are non-existent.
All physics
revolves around the concept of RELATIVE velocity, that is,
how rapidly the
distance between you and some reference object changes
in a certain time
interval. Velocity does exist as a concept, but only
its relative character
has any physical meaning.